Modern ink-jet printers, for example, the Epson Photo EX, are capable of generating images of so called photographic quality. In order to realise the full potential of these printers, it is necessary to use special media on which to print. Some of the essentially required properties for such media are best understood by considering the failure modes of plain paper, typical coated papers and coated films.
1. Plain Paper
When plain paper is used as the receiver medium in an ink-jet printer, it is found that coloured inks penetrate into the paper such that the colour is partially obscured by light scattered by the paper fibres. Additionally, ink is spread laterally by capillary action along the fibres, leading to distortion of the coloured spots that form the image, and thereby to distortion of the image itself. In addition to these faults, the paper is found to distort, sometimes referred to as cockle, by the action of the water present in the ink in the regions of the image that carry a heavy ink loading, or by accidental immersion in water. Finally, plain paper is not glossy because of the rough nature of the fibre surface.
2. Coated Papers
Many of the above defects can be overcome by applying special coatings to the surface of the paper. However, in these products there remains the problem of cockle as referred to above. This is always true for coatings that are permeable to water.
3. Coated Films
The term “coated films” is used to refer to both plastics substrates and papers that are laminated or coated with plastics materials, such that they are not permeable to inks or water, thus preventing cockle. The coatings on these films may be either porous or non-porous. Non-porous coatings may absorb the ink to a greater or lesser extent by a polymer swelling mechanism. Those coatings that swell to a lesser extent are not capable of absorbing all the ink in heavily inked areas and therefore exhibit slow ink drying times after printing, because the print will only fully dry when excess water has evaporated. Those coatings which swell to a greater extent tend to be water soluble, or are easily damaged mechanically, and this limits image stability. WO 99/21724 concerns an ink-jet printing medium comprising a non-porous coating which absorbs water from the applied ink by polymer swelling. Porous coatings have the potential for overcoming these problems by fast absorption of the ink by a controlled uniform capillary process. However, it has been found that there are practical difficulties in achieving a glossy, non-cracked porous coating. Polymeric binders are necessary to achieve acceptable strength in the coating and also to assist in fixing the colorants in the ink. However, it is generally found that the incorporation of polymeric binders in such structures either leads to a non-glossy finish on the one hand or to a slowly absorbing coating on the other.